Although nonclassical (class I-b) gene products represent the majority of murine major histocompatibility complex (MHC) genes, the role of these relatively nonpolymorphic molecules remains uncertain. Recently, one such protein, H-2M3 (formerly designated Hmt), was shown to bind and specifically present N-formylated peptides to cytotoxic T lymphocytes. Because N-formylation is characteristic of prokaryotic proteins, this MHC molecule may be especially adapted for a role in the mammalian defense against bacterial attack. The current studies demonstrate that an MHC molecule, indistinguishable from H-2M3, presents antigens derived from the intracellular pathogen Listeria monocytogenes to Listeria-specific CD8+ cells.